Means for obtaining multiple coherent-laser apertures

ABSTRACT

Multiple coherent-laser apertures produced by directing laser beams through a prism for optical interaction and coherence.

United States Patent [72] Inventor George R. Simpson Webster, N.Y.

[2]] Appl. No. 759,594

[22] Filed Sept. 13, 1968 [45] Patented May 4, 1971 [73] Assignee TheUnited States of America as represented by the Secretary of the Army[54] MEANS FOR OBTAINING MULTIPLE COHERENT- LASER APERTURES 6 Claims, 5Drawing Figs.

[52} U.S. Cl 331/945 H015 3/05 [50] Field of Search 331/965 [56]References Cited UNITED STATES PATENTS 3,312,905 4/1967 Lewis 331/945Primary Examiner-Roy Lake Assistant Examiner-Darwin R. HostetterAttorneys-Harry M. Saragovitz, Edward J l Kelly, Herbert Berl and Aubrey.I. Dunn ABSTRACT: Multiple coherent-laser apertures produced bydirecting laser beams through a prism for optical interaction andcoherence.

PATENTEB MAY 4197:

George R. Simpson,

INVENTOR.

MEANS FOR OBTAINING MULTIPLE COIIERENT- LASER APERTURES BACKGROUND OFTHE INVENTION This invention is in the field of lasers. In particular.this invention relates to the direction of laser beams through prisms toachieve optical interaction.

There is a need in many applications of lasers to increase the apertureof the beam so more laser power may be incident on a target. There is apractical limit to the aperture that can be obtained from a single laserrod. The aperture is limited by two factors. (1) If the laser rod has alarge cross-sectional area, spontaneous emission in directions otherthan the desired direction determined by the end reflectors depopulatesthe inverted ions, so that they are not able to contribute their energyto the desired mode of operation. (2) If the crosssectional area of thelaser rod is too large, the majority of the pumping energy is absorbedat the periphery of the laser rod. Consequently, the inversion in thecentral volume of the rod is too small to induce laser action.

Presently, the only technique of introducing more volume to a coherentlaser device is to restrict the aperture as determined above and toincrease the length. The coherent aperture of any device, consequently,is limited by the conditions specified above and by the limit fordecreasing the beam spread.

There are definite realistic limits for increasing the laser intensitythrough the above means. To further increase intensity, multiple laserbeams must be combined. This invention provides a method for combining aplurality of laser beams to achieve a resultant having a coherentaperture.

SUMMARY OF THE INVENTION Two laser beams are focused to coincide on thebase of a prism. The prism is oriented so that the base of the prismbisects the angle formed by the two laser beams. Part of each beam isreflected into the other by the base of the prism. The vertex angle andthickness of the prism determines the angle at which the coherent beamsemerge. Proper selection of these variables enables the beams to emergeparallel and adjacent each other. The resulting coherent aperture istwice that of a single rod.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a schematic diagram of apreferred embodiment of the present invention;

FIG. 2 is an end view of FIG. 1 taken about line 2-2.

FIG. 3 is a schematic diagram of another embodiment of the presentinvention;

FIG. 4 is an end view of FIG. 3 taken about line 4-4; and

FIG. 5 is a schematic diagram of yet another embodiment of the presentinvention.

DESCRIPTIONOF THE PREFERRED EMBODIMENT Referring to FIG. I, laser rodsand 12 are independently pumped by conventional means 8. The laser rodshave a bevel 14 on one end of the rod and a roof prism 16 on the otherend. The bevel is placed on the rods so that its Fresnel reflection andthe roof prism do not form a Fabry-Perot-type cavity which would resultin independent laser action in each rod. The two rods are spaciallyoriented so that their beams 18 and 20 coincide at the base facet 21 ofthe pentaprism 22. Pentaprism 22 is oriented so that the normal to thepentaprism base facet 21 bisects the angle formed by the two laser beams18 and 20. Part of the power emitted by each rod will be Fresnelreflected by the prism base facet into the other. Thus, aFabry-Perot-type cavity having end reflectors 16 is formed via theFresnal reflections. This optical coupling between the two laser beamscauses the radiation output of each from the prism to be coherent withthat of the other. The vertex angle 24 and thickness 26 of the prism 22are adjusted so that the coherent beams 28 and 30 emerge from the prismat virtually any angle desired. The two laser beams of FIG. 1 emergeparallel coherent, and adjacent. The resulting coherent aperture 32(FIG. 2) is the sum of the apertures of the single rods III and 12. or amultiple ofthe aperture ofa single laser rod.

If it is desired to enlarge the aperture 32 of FIG. 2. the basicconfiguration of FIG. 1 may be stacked as shown in FIG. 3. The basicconfiguration of FIG. I comprises laser rods 10 and I2. pentaprism 22.and pumping means 8. Two of these configurations are shown in FIG. 3arranged to produce an aperture 34 (FIG. 4) twice as large as theaperture of FIG. 2.

Coherence between the laser beams within each basic configuration is thesame as it was in the preferred embodiment shown in FIG. 1. Coherencebetween the beams of the basic configurations is achieved by the samemethod as was coherence between the individual laser beams of the basicconfiguration. Coherence between the beams 36 and 38 emerging from thebasic configurations is created by the Fresnel reflections coupling ofprism base 35 of pentaprism 37. It is apparent that the basicconfigurations may be stacked in various arrangements to achieve adiversified arrangement of coherent laser apertures.

Another form of this invention is shown in FIG. 5. This embodimentyields multiple coherent apertures that emit planepolarized laserradiation. Laser rods 40 and 42 are pumped by conventional means 43 andhave a roof prism 44 on one end and a Brewster's angle bevel 46 on theother end. The laser energy emitted from the rods will inherently beplane polarized because of the Brewster's bevel. The laser rods areoriented so that the laser beams 48 and 50 are coincident on the basefacet 52 of pentaprism 54 and are incident at the pentaprisms Brewster'sangle. The rods are also oriented so that the E-vector of the polarizedlaser energy is normal to the plane of the drawing. The laser energy inthe plane of polarization, with the E-vector vibrating normal to theplane of the drawing, is refracted through the Brewsters bevel withoutany Fresnel reflection. The laser polarization parallel to the plane ofthe drawing is nonexistent because there is zero Fresnel reflection atthe prism base for this polarization. Thus, a Fabry-Perot-type cavityfor this polarization is not established and no laser action occurs.

It will be apparent to those skilled in the art that changes may be madein the form of the apparatus disclosed without departing from the spiritof the invention as set forth in the appended claims, and that in somecases certain features of the invention may sometimes be used toadvantage without a corresponding use of the other features.

lclaim:

1. In combination: a plurality of laser-emitting rods and a pentaprismhaving a cross section comprising generally an A- shaped roof, twosides, two ends, and a base. said rods being oriented so that theirrespective laser beams are coincident on said base of said prism, saidlaser beams being refracted by said prism and emerging from said prismto form a coherent laser aperture proportional to the sum of the laserenergy emitted by said rods, said laser-emitting rods having a roofprism on a first end and a bevel angle on a second end. said bevel beingplaced on said rod to produce coherent laser action between said rodswherein said laser beams coincident on said base are Fresnel reflectedby said base into each opposite rod to form a Fabry-Perot-type cavityhaving end reflectors formed by said roof prism on said first end ofsaid rods.

2. The invention as set forth in claim 1 wherein the longitudinal axesof said laser-emitting rods lie in the same plane.

3. The invention as set forth in claim 2 wherein a line normal to saidside of said prism lies in said plane and bisects the angle between saidlaser beams.

4. The invention as set forth in claim I wherein said bevel is aBrewsters angle bevel.

5. The invention as set forth in claim 4 wherein said laser beams areplane polarized and wherein the E-vector of said plane polarized laserbeams is parallel to the longitudinal axis of said prism, said laserbeams being incident on said side of said prism at the Brewsters angleof said prism.

6. The invention as set forth in claim 1 wherein said laser beamsemerging from said prism are parallel, adjacent and coherent.

2. The invention as set forth in claim 1 wherein the longitudinal axesof said laser-emitting rods lie in the same plane.
 3. The invention asset forth in claim 2 wherein a line normal to said side of said prismlies in said plane and bisects the angle between said laser beams. 4.The invention as set forth in claim 1 wherein said bevel is aBrewster''s angle bevel.
 5. The invention as set forth in claim 4wherein said laser beams are plane polarized and wherein the E-vector ofsaid plane polarized laser beams is parallel to the longitudinal axis ofsaid prism, said laser beams being incident on said side of said prismat the Brewster''s angle of said prism.
 6. The invention as set forth inclaim 1 wherein said laser beams emerging from said prism are parallel,adjacent and coherent.